Online service providers offer many products and services to their clients. One such service is the ability of a client to purchase and operate a certain amount of computing capacity on a remote computer system. For example, a client may submit a request to create and operate a virtual web server running on a host server of the online service provider. Typically, the instantiation of computing capacity on a computer system involves the creation and execution of a machine image. The process of selecting, creating, and executing a machine image, however, is often cumbersome and inefficient.